OK, I'll assume that the water is not just in the reservoir tank, but completely into the system? This is not a small job - not particularly hard, but it is going to take some time. I'm sure there's more than one way to do this, but I would start by removing the cylinders and taking them apart so you can fully clean them out and flush with Brakleen, then also flush with new 30wt to get all the water out. Pre-fill the cylinders (this will help to cut down on the re-bleeding time). Next, with everything still disconnected on the outside, remove the lines from the pump and blow them out. Remove trim pump assembly from reservoir and clean it out real well. There may still be some left in the valve body, but that should flush out quickly. Reinstall, add fluid, bleed system. It will take many, many times to fully bleed. Keep going until all bubbles are gone in reservoir and drive feels solid. When you get bubbles built up in the reservoir, you'll need to wait until they go away before continuing - this will be time consuming.

Something I've never tried, but might be worth a shot as it could make things go a little faster (if it works): After cleaning the reservoir and blowing the lines out... reattach the lines on the pump only (not to the cylinders) and fill the reservoir. Now, take the down line that's on the outside of the boat and stick it into a new quart of 30wt. Stick the up line into a junk container. Activate the trim pump to try and "prime" the lines. IF that works, it will get rid of a lot of the air in the system. If you do this, you're my guinea pig - let me know how it works! You won't damage anything - worst case is that it doesn't suck the oil into the down line. You can monitor the reservoir, too.